anybody use a vacuum gauge on their cruiser?

i have one hooked up and have noticed an odd needle behavior that i am wondering if anyone else has seen.

at wide open throttle, or nearly so, i notice the vacuum gauge flutters across a range of 2-4 inches of vacuum. if i push the gas just a little more, it quits fluttering and i am usually pegged at 0 inches. backing off the gas usually brings back the flutter until i am not under so much load, usually at a vacuum of 5-7 inches.

what does the fluttering translate to? i was thinking maybe it was a point where the valves are just not keeping up with the demand as much, maybe bouncing?

Although the page linked to is a great resource for anyone beginning to appreciate the wonders of vacuum readings it only touches your problem (if it actually IS a problem).
I think that you're seeing the influence of emmission regulations. The engineers brought you a lean burn condition at mid throttle - just the thing for passing parts of some required testing when seeking approval for the sale of a new vehicle.
Although you might be able to get the gauge to steady at midrange by using the high altitude jets included with your carburator, I doubt that there's much of a drivability problem for you, right? The truck runs OK and this gauge fluctuation is your first indication of something awry?

Jim c or another good carb guy could probably tune away that reading for you if you want, but if you aren't having problems driving your truck as it is, then apply the "If it works Don't fix it" maxim and leave it be.

I just installed one on the dash along with mech temp and oil pressure. I've only run the new motor about 20 minutes because my fun-hating 85 year old neighbor does not have an appreciation for open headers.....prick.
However a cab-mounted vacuum gauge is a requirement if you are trying to maximize your fuel economy through driving habits. I also have one that I use under the hood for idle adjustments and timing.
As to the fluctuations, I can't add anything, but what Honk says seems to make sense with regard to the emissions junk. Mine is old enough that I don't have any.

the rig does run fine, it is just a quirk i noticed early on since the rebuild and i was just curious what it might mean.

i talked with my local machinist who is building my 2FE and he confirmed much of what i have seen out there. fluttering gauge usually indicates weak springs. he had mentioned that the F series motors generally have weak springs, usually because they arent run at high enough RPM or hard enough to make firmer springs worth the effort. firm springs put more wear on your cam and lifter assembly, so toyota went with weak ones.

ultimately, it isnt a big deal as long as it isnt consistently happening. valve float, as he called it, can easily be eliminated if you back off the gas just a bit. beyond loosing compression/power from it, he had said it wasnt much of a deal to worry about since i am running a tractor on the freeway.

he also assured me that my 3FE head with new chev oversize valves would not suffer from valve float as it has new Z28 springs, a much stronger spring...

oh, and i too would recommend a vacuum gauge if you are trying to optimize your fuel economy. shoot for 14-15 inches at speed and you will see a big difference at the pump! it also helps keeps you informed with regards to your carb performance!